Nanotechnology Spotlight

Stretchable energy storage and conversion

(Nanowerk Spotlight) Stretchable electronics are mechanically robust electronics which can be bent, folded, crumpled and stretched and are a major research focus of nanotechnology scientists towards next-generation wearable and implantable devices. Unlike today's electronics based on rigid silicon technologies, stretchable devices can conform to almost any kind of surface shapes and provide unique functionalities which are unreachable with simple extension of conventional technologies.

Stretchable energy storage and conversion devices are key components for the fabrication of complete and independent stretchable systems. In a recent review in Small ("Stretchable Energy Storage and Conversion Devices"), researchers summarize the recent progresses in the developments of stretchable power sources including supercapacitors, batteries and solar cells.

They discuss representative structural and material designs to impart stretchability to the originally rigid devices and analyze advantages and drawbacks associated with the various fabrication methods. They also present summaries of the research progresses along with future development directions.

Stretchable electrodes

As one of the key components for stretchable devices, the electrodes have to be able to withstand such harsh mechanical requirements. Several strategies have been used to fabricate stretchable electrodes, and in general can be categorized into the following two groups: Structures that stretch; Materials that stretch.

Creating structures that can be stretched is the most successful strategy for stretchable electronics, and the stretchable structures can be categorized into the following four types: Non-coplanar buckled structures; Coplanar serpentine structures; Percolating nanostructured films; Textiles.

Materials that stretch are mainly based on polymers with flexible and stretchable molecular chains. Composites based on organic polymers and inorganic structures such as nanowires and nanoparticles are categorized as stretchable structures since functional inorganic components are rigid and the stretchability originates from the percolating structure.

Stretchable supercapacitor

Most commonly used active materials for double-layer supercapacitors are carbon-based materials such as single-walled carbon nanotubes (SWCNTs), graphene and mesoporous carbon, due to their excellent electrical conductivity and high surface areas. The carbon-based materials are also natural choices for stretchable supercapacitors, with proper structural engineering to impart stretchability to the devices (see for instance: "Power sources for flexible, stretchable electronics").

Stretchable double-layer supercapacitors based on SWCNTs films on both elastic PDMS substrates and textile substrates have excellent stretchability, electrochemical and mechanical stability. However, the key drawback of those double-layer devices is the low specific capacitance and energy density and this can potentially be improved by the incorporation of redox materials.

Compared with inorganic solar cells, soft organic solar cells usually have inferior performances and poor environmental stability. But they are advantageous in terms of low costs, structural flexibility and ease of production from solution processes. The soft, flexible and stretchable characteristics of organic polymers allow ready construction of stretchable solar cells.

The authors conclude their review with the forecast that continuous developments will go from individual discrete components and devices (such as stretchable power sources) to integrated complex systems. "With the growing interest from both academic and industrial areas, breakthroughs in discrete components and devices can be readily expected. Integration and coordination of those functional devices into complete systems will pave the way for practical applications."